High potential thermoelectric figure of merit in ternary La₃Cu₃X₄ (X = P, As, Sb and Bi) compounds

We investigate the thermoelectric properties of the relatively unexplored rare-earth ternary compounds La₃Cu₃X₄ (X = Bi, Sb, As, and P) using first principles electronic structure and Boltzmann transport calculations. These compounds, of which the La₃Cu₃Sb₄ and La₃Cu₃Bi₄ have previously been synthesized, are all predicted to be semiconductors and present a wide range of bandgaps varying from 0.24 eV (for the Bi compound) to 0.87 eV (for the P compound). We further find a mixture of light and heavy bands, which results in a high thermoelectric power factor. In addition, as discussed in our previous study (Phys. Rev. B 95 (22), 224306, 2017) at high temperatures of 1000 K these compounds exhibit lattice thermal conductivity less than 1 W/mK. The combination of low thermal conductivity and good transport properties results in a predicted ZT as high as ~1.5 for both La₃Cu₃P₄ and La₃Cu₃As₄, under high p-type doping. This predicted high performance makes these compounds promising candidates for high temperature thermoelectric applications and thus merits further experimental investigation.